1. Field of the Invention
This invention relates generally to hot-melt compositions and, more particularly, to hot-melt compositions based on ethylene/vinyl acetate (EVA) copolymers, ethylene/n-butyl acrylate (EnBA) copolymers and tackifying resins. Hot-melt compositions according to the invention have good open time and form creep-resistant bonds when applied in thin layers.
2. Description of the Related Art
Hot-melt compositions are generally understood to comprise thermoplastic materials which are solid at room temperature but which readily melt to a viscous liquid upon heating. The molten composition is applied to a substrate and the substrate is mated to a second surface. As the composition cools, it rapidly solidifies and then crystallizes or "sets up" thereby forming a bond between the substrate and the second surface. Accordingly, the term "set time" has been adopted to define the time required for the hot-melt composition to crystallize and the bond between the substrate and the second surface to form after the substrate and the second surface have been mated.
"Set time" is to be distinguished from "open time" (sometimes referred to herein as "tack bonding range") which refers to the time over which the hot-melt composition remains tacky after having been applied to an unheated, room temperature (about 20.degree. to 22.degree. C.) substrate. Once the "open time" has been exceeded, the hot-melt composition becomes tack-free and the substrate and the second surface cannot be successfully mated unless the composition is capable of being reactivated (i.e., the composition can be rendered tacky again).
Typical uses for hot-melt compositions include the high speed, automated assembly of low strength products such as in bookbinding and packaging operations. Hot-melt compositions have traditionally been applied to a substrate as a bead. When applied in bead form, presently known hot-melt compositions may offer two advantageous properties, an open time of at least 5 seconds coupled with a relatively short set time. An open time of at least 5 seconds allows ample time for mating the surfaces to be bonded while a short set time indicates that the bond has crystallized to the extent that the mated surfaces no longer need to be clamped or held together. A short set time permits bookbinding and cardboard carton assembly equipment to be operated at maximum speed.
Another characteristic demonstrated by such compositions is their ability to form "creep resistant" bonds. "Creep resistance" refers to the capacity of a bond to sustain a constant load under a constant stress. A bond which "creeps" moves slowly when under stress. Items bonded together by an adhesive which tends to "creep" will move, may not stay in place and may even fail. A bond which is "creep-resistant" combats this tendency.
However, it is not always desirable to apply a hot-melt composition as a bead. When a bead of adhesive is deposited on a substrate and a second surface is mated thereto, the bead is compressed and spreads out between the substrate and the second surface. In some instances, the composition may be squeezed out beyond the edges of the mated surfaces creating a messy, unaesthetic appearance. Alternatively, the composition may not fully extend to the edges of the mated surfaces thereby creating a situation referred to as "edge gap" which increases the opportunity for delamination of the substrate and the second surface. In general, when a hot-melt composition is distributed between a substrate and a second surface by compressing a bead of the composition, it is difficult to obtain a layer of the composition which uniformly covers the substrate and which is of substantially equal thickness throughout. Failure to achieve such a distribution is particularly problematic when bonding relatively thin substrates since the uneven application of the hot-melt composition may be telegraphed through the substrate (i.e., the uneven application of the hot-melt composition becomes visually apparent).
The application of hot-melt compositions as beads has been associated with other undesirable occurrences. Beads may wick through the substrate, especially if the substrate is thin. Also, a bead may indelibly stain a porous substrate.
Accordingly, there is considerable need for a hot-melt composition which may be usefully applied to a substrate in a thin layer. Such a hot-melt composition would offer extraordinary advantages over hot-melt compositions which are applied as beads. For example, the composition could be uniformly applied to the entire surface of a substrate in a layer of substantially constant thickness. The likelihood that the composition would be squeezed out beyond the edges of the substrate or would not fully extend to the edges thereof would be greatly reduced. Similarly, a thin layer would be less likely to telegraph through, wick through, or permanently stain a substrate.
Presently known hot-melt compositions, when spread as a thin layer on a room temperature substrate, do crystallize to form creep-resistant bonds. Unfortunately, these compositions also exhibit a very short open time on the order of 1 to 2 seconds. While an open time of such short duration may be acceptable in certain applications, such as some high speed, automated cardboard carton sealing operations where the surfaces to be mated can be brought together within 1 to 2 seconds of the adhesive having been deposited, in other instances it is desirable to have an open time of at least 5 seconds. For example, in the construction of signs, advertising displays and other articles having a relatively large surface area (as compared to the sealing of cardboard cartons), a relatively long open time adhesive may be required so as to provide ample time in which to complete the assembly of the article while the adhesive remains tacky.
In other industries such as lithography, die cut parts and costume jewelry, the two parts which are to be adhesively bonded must be carefully and precisely positioned with respect to each other. An adhesive with an open time of at least 5 seconds provides the assembler of such components sufficient time to accurately position the bonded parts and, if necessary, to reposition them until they are properly aligned. The 1 to 2 seconds of open time afforded by presently known hot-melt adhesives when spread in a thin layer is simply too short to allow for precision placement. (The adhesive becomes nontacky before the components can be properly positioned.)
Pressure sensitive adhesives are a class of materials which are essentially permanently tacky at room temperature and may be regarded as having a virtually infinite open time. Intuitively, materials having a virtually infinite open time would appear to be well-suited to the construction of large surface area articles or precision components because of the long time during which the construction of these articles could be completed. However, such is not the case. As the open time of an adhesive increases, the opportunity for dirt and other materials to contaminate the adhesive surface is enhanced because the tacky adhesive readily retains the same. Contaminants are asthetically displeasing, especially in articles such as costume jewelry. More importantly, contaminants deposited on the adhesive surfaces prior to bonding can weaken the ultimate strength of the bond by reducing the available surface area of the adhesive.
The virtually infinite open time of a pressure sensitive adhesive can pose long term storage and inventory management problems too. For example, the manufacture of certain articles such as signs and advertising displays may involve laminating several panels together to provide the finished article. The number and types of panels required will depend on the ultimate size of the articles, where the articles are to be placed, and customer preferences. Consequently, it may be desirable for a manufacturer of such articles to maintain an inventory of panels which have been precoated with an adhesive to facilitate the rapid assembly of the finished articles upon customer demand. However, if these panels are coated with a pressure sensitive adhesive, the essentially permanently tacky surface will readily retain dust and dirt, rendering the useful storage of precoated panels virtually impossible. Also, the precoated panels can not be conveniently stacked (unless expensive and unwield release liners are employed) since they will adhere to adjacent panels in the stack.
Consequently, it would be desirable for the adhesive used to precoat such panels to have a limited open time such that the panels could be readily prepared in advance and stored. Of course, the adhesive would have to be capable of being reactivated (i.e., restored to a tacky condition) so that assembly of the finished articles could be subsequently completed.
Furthermore, pressure sensitive adhesive compositions (even those which are considered hot-melt pressure sensitive adhesives) do not form creep-resistant bonds when spread in a thin layer. As noted above, items bonded together by an adhesive which tends to creep will move, may not stay in place, and may even fail. In the bonding of precision components, a creep-resistant adhesive helps prevent movement which would otherwise disturb the accurate positioning of the components.
Thus, it would be desirable for a hot-melt composition when spread in a thin layer to have a limited open time; that is, an open time greater than about 5 seconds (so as to exceed conventional hot-melt compositions) but which is not virtually infinite (so as to decrease the opportunity for dirt contamination as happens with conventional pressure sensitive adhesives).
Hot-melt compositions are often applied with dispensers commonly referred to as "glue guns." In these devices, the composition is provided in the form of a relatively hard, rigid stick or rod which is pneumatically or mechanically driven into an electrically heated well or chamber at the rear of the device. Most presently known compositions require that the well be heated to a temperature of at least about 350.degree. to 400.degree. F. (177.degree. to 204.degree. C.) to melt the composition and to reduce its viscosity. The molten composition is forced through a barrel-like portion of the device and exits from the opposite end thereof to be applied to a substrate. The temperature of the molten composition upon discharge is typically about 330.degree. to 380.degree. F. (166.degree. to 193.degree. C.).
Hot-melt compositions which must be applied at a temperature of about 350.degree. to 400.degree. F. (177.degree. to 204.degree. C.) pose unique problems whether deposited as a bead or a thin film. For example, at these temperatures, a negligent user of the hot-melt composition may be seriously burned by contact with the same. Also, such compositions cannot be used in conjunction with relatively heat sensitive substrates (e.g., expanded polystyrene) as the same may melt. Furthermore, such hot-melt compositions can be more difficult to apply. Because of the relatively high melting temperatures, a large power supply is needed to heat the glue gun chamber so as to melt the adhesive rod, thereby limiting the ability to use a portable power supply (such as batteries, for example). Similarly, the time required to heat the glue gun to the proper operating temperature is increased.
The use of EVA copolymers in hot-melt applications is mentioned in several references. U.K. Pat. Spec. No. 1,233,797 to Cheetham et al., published May 26, 1971, states: "Known hot-melt adhesives for bonding paper stocks are 100% solids systems consisting of, for example, a mixture of an ethylene/vinyl acetate copolymer, a resin and a wax. . . . It has been found, however, that bonds between paper substrates formed with these adhesives exhibit unacceptable creep or part completely at temperatures of about 50.degree. C. Packs made under such adhesives fail under tropical storage conditions" (Page 1, lines 15-38.)
By 1980, as reported in Duncan et al., "EVA 7 and EVA Copolymers for Hot-melt PSA's," Adhesives Age, March 1980, pp 37-41: "Ethylene/vinyl acetate (EVA) copolymers are well established as the base polymer for hot-melt package sealing adhesives." Duncan et al. disclose that EVA copolymers may be blended with various tackifying resins so as to provide pressure sensitive adhesives. The open time or tack bonding range and creep resistance of the resulting bonds are not discussed, but the article concludes that EVAs in combination with ester resins or aromatic hydrocarbons "can be formulated in hot-melts suitable for pressure-sensitive tapes and labels but appear to lack the shear strength required for structural applications."
U.S. Pat. No. 4,613,632 to Aliani et al. discloses an EVA-based hot-melt adhesive that includes a resin which functions as a tackifier and which also assists wetting. The Aliani et al. adhesive may contain a wax but preferably is wax-free. The EVA has a vinyl acetate content of from 11% to 40% by weight and a melt index of from 700 to 10,000 in a wax-free hot-melt system.
Other presently known references mention the use of EnBA copolymers in hot-melt applications. For example, "EnBA Resins Give Hot Melts More Heat and Oil Resistance" by D. Wielinski (Adhesives Age, November 1989, p. 30, et. seq.) notes that n-butyl acrylate may be substituted for vinyl acetate in an ethylene/vinyl acetate composition with consequential improvement in certain resin properties such as enhanced low temperature flexibility, oil resistance and heat stability. It is stated that n-butyl acrylate, as compared to vinyl acetate, can be employed at higher application temperatures, and has broader compatibility with a variety of tackifiers and waxes.
The article only discusses EnBA copolymers having a melt index of 400 or less. There is no indication that EnBA copolymers with a melt index in excess of 400 would be useful. Higher melt index copolymers, relative to those with a lower melt index, are capable of being applied at a lower temperature. As discussed more fully above, numerous advantages can be realized by hot-melt compositions with lower application temperature. Furthermore, higher melt index copolymers tend to be less viscous at a given temperature and, as a result, flow more easily and provide better substrate wetting. In general, these compositions are easier and safer to use.
U.S. Pat. No. 4,816,306 and its division U.S. Pat. No. 4,874,804, "Hot Melt Adhesive Based on Ethylene-N-Butyl Acrylate," each to Brady et al., disclose hot-melt adhesives based on EnBA copolymers. These patents disclose hot-melt compositions consisting essentially of 35% to 45% by weight of an EnBA copolymer, 35% to 55% of a terpene phenolic tackifier, and 10% to 20% by weight of a high melting synthetic wax that are especially useful in high speed, rapid-fire automated cardboard case and carton sealing equipment. The adhesive compositions were applied as a bead at 175.degree. C. and subsequently compressed.
U.S. Pat. No. 3,615,106, "Manufacture of Hard Covered Books," issued Oct. 26, 1971 to Flanagan et al., discloses a hot-melt adhesive composition comprising 38% EnBA, 20% paraffin wax, 35% tackifying resin, and 7% polyethylene. The composition is useful in producing hard bound books. Preferably the EnBA has a melt index in the range of from about 1.0 to 20.0. The compositions are applied at temperatures of from about 300.degree. to 400.degree. F. to provide 5 to 30 mil thick wet coatings.
Thus, there is a need for a hot-melt composition which, when spread in a thin layer, has a limited open time of at least 5 seconds. Compositions with an open time of at least 5 seconds are needed when bonding large surface area substrates and precision components. However, the open time of such compositions should be limited. That is, they should not have a virtually infinite open time as do pressure sensitive adhesives so as to reduce the likelihood of dirt contamination on the edges of a substrate and the deterioration of bond strength which can occur if dirt contaminates the tacky substrate before bonding.
It would be advantageous if such compositions were creep-resistant, especially if used to bond precision components.
Several other characteristics would further contribute to the desirability of the hot-melt compositions. For example, it would be preferred if bonds could be formed without applying more than hand pressure or without supplying more heat than is generated by the application of hand pressure. A composition having such bond formation characteristics would be remarkably easy to use. Large, cumbersome heated presses would not be required to assemble components.
Presently known hot-melt compositions based on either EVA copolymers or EnBA copolymers do not demonstrate both a limited open time of at least 5 seconds and creep resistance when deposited as thin films on unheated (i.e., room temperature) substrates. Moreover, those compositions which are presently known do not exhibit these characteristics in combination with the ability to form bonds without applying heat or more than hand pressure.